Mechanical and chemical-mechanical planarization processes (collectively “CMP”) are used in the manufacturing of electronic devices for forming a flat surface on semiconductor wafers, field emission displays and many other microelectronic-device substrate assemblies. CMP processes generally remove material from a substrate or substrate assembly to create a highly planar surface at a precise elevation in the layers of material on the substrate. FIG. 1 schematically illustrates an existing web-format-planarizing machine 10 for planarizing a substrate 60. The planarizing machine 10 has a support table 14 with a sub-pad 16 at a workstation where an operative portion “A” of a planarizing pad 40 is positioned. The sub-pad 16 is generally a rigid plate to provide a flat, solid surface to which a particular section of the planarizing pad 40 may be secured during planarization.
The planarizing machine 10 also has a plurality of rollers to guide, position and hold the planarizing pad 40 over the sub-pad 16. The rollers include a supply roller 20, idler rollers 21, guide rollers 22, and a take-up roller 23. The supply roller 20 carries an unused or pre-operative portion of the planarizing pad 40, and the take-up roller 23 carries a used or post-operative portion of the planarizing pad 40. Additionally, the left idler roller 21 and the upper guide roller 22 stretch the planarizing pad 40 over the sub-pad 16 to hold the planarizing pad 40 stationary during operation. A motor (not shown) drives at least one of the supply roller 20 and the take-up roller 23 to sequentially advance the planarizing pad 40 across the sub-pad 16. Accordingly, clean pre-operative sections of the planarizing pad 40 may be quickly substituted for used sections to provide a consistent surface for planarizing and/or cleaning the substrate 60.
The web-format-planarizing machine 10 also has a carrier assembly 30 that controls and protects the substrate 60 during planarization. The carrier assembly 30 generally has a substrate holder 50 to pick up, hold and release the substrate 60 at appropriate stages of the planarizing process. The substrate holder 50 engages a retainer ring 31 that surrounds the microelectronic substrate 60 and restricts lateral motion of the microelectronic substrate 60 relative to the substrate holder 50. Several nozzles 33 attached to the substrate holder 50 dispense a planarizing solution 44 onto a planarizing surface 42 of the planarizing pad 40. The carrier assembly 30 also generally has a support gantry 34 carrying a drive assembly 35 that can translate along the gantry 34. The drive assembly 35 generally has an actuator 36, a drive shaft 37 coupled to the actuator 36, and an arm 38 projecting from the drive shaft 37. The arm 38 carries the substrate holder 50 via a terminal shaft 39 such that the drive assembly 35 orbits the substrate holder 50 about an axis B-B (as indicated by arrow “R1”). The terminal shaft 39 may also rotate the substrate holder 50 about its central axis C-C (as indicated by arrow “R2”).
The planarizing pad 40 and the planarizing solution 44 define a planarizing medium that mechanically and/or chemically-mechanically removes material from the surface of the substrate 60. The planarizing pad 40 used in the web-format planarizing machine 10 is typically a fixed-abrasive planarizing pad in which abrasive particles are fixedly bonded to a suspension material. In fixed-abrasive applications, the planarizing solution is a “clean solution” without abrasive particles. In other applications, the planarizing pad 40 may be a non-abrasive pad without abrasive particles. The planarizing solutions 44 used with the non-abrasive planarizing pads are typically CMP slurries with abrasive particles and chemicals.
To planarize the substrate 60 with the planarizing machine 10, the carrier assembly 30 presses the substrate 60 against the planarizing surface 42 of the planarizing pad 40 in the presence of the planarizing solution 44. The drive assembly 35 then orbits the substrate holder 50 about the axis B-B and optionally rotates the substrate holder 50 about the axis C-C to translate the substrate 60 across the planarizing surface 42. As a result, the abrasive particles and/or the chemicals in the planarizing medium remove material from the surface of the substrate 60.
The CMP processes should consistently and accurately produce a uniformly planar surface on the substrate 60 to enable precise fabrication of circuits and photo-patterns. During the fabrication of transistors, contacts, interconnects and other features, many substrates or substrate assemblies develop large “step heights” that create a highly topographic surface across the substrate assembly. Yet, as the density of integrated circuits increases, it is necessary to have a planar substrate surface at several intermediate stages during the fabrication of devices on a substrate assembly because non-uniform substrate surfaces significantly increase the difficulty of forming sub-micron features. For example, it is difficult to accurately focus photo patterns to within tolerances approaching 0.1 micron on non-uniform substrate surfaces because sub-micron photolithographic equipment generally has a very limited depth of field. Thus, CMP processes are often used to transform a topographical substrate surface into a highly uniform, planar substrate surface.
One problem with some conventional CMP techniques is that the carrier assembly 30 may not apply a uniform downward force on the substrate 60. Accordingly, the planarized surface of the substrate 60 may develop non-uniformities that can adversely affect subsequent processing steps. One approach to address this problem is to rotate the substrate 60 and the substrate holder 50 as a unit about axis C-C, but a drawback with this approach is that it increases the mechanical complexity of the substrate holder 50 and the carrier assembly 30. This approach particularly increases the mechanical complexity if the carrier assembly 30 includes fluid couplings between the arm 38 and the substrate holder 50 (for example, to supply the planarizing liquid 44 to the nozzles 33).
Another problem with some conventional devices is that the retainer ring 31 can wipe the planarizing liquid 44 from the planarizing pad 40 as the substrate carrier 50 and the substrate 60 move across the planarizing pad 40. Accordingly, the planarizing liquid 44 may not uniformly coat the lower surface of the substrate 60, which can reduce the planarizing rate and/or the planarity of the substrate 60. Still another problem is that the carrier assembly 30 can include a disposable film at the interface between the substrate holder 50 and the substrate 60 to grip the substrate 60. The disposable film must be periodically replaced, which increases the time and expense required to maintain the apparatus 10.
One approach for addressing some of the foregoing problems is to direct pressurized air against the rear surface of the substrate 60 during planarization. For example, U.S. Pat. No. 5,762,539 to Nakashiba et al. discloses a carrier apparatus that directs several jets of compressed air at different pressures toward the rear surface of the substrate to bias the substrate against the polishing pad while the carrier rotates relative to the polishing pad. One drawback with this approach is that the arrangement for supplying compressed air to the carrier may be complex and subject to leaks because it includes rotary couplings that direct the compressed air to the rotating carrier. A further disadvantage is that the compressed air can evaporate the planarizing liquid, reducing the effectiveness of the planarizing medium. When the planarizing liquid includes an abrasive slurry, the compressed air can evaporate the liquid portion of the slurry, causing the abrasive particles in the slurry to agglomerate. Furthermore, the thickness of the air cushion between the carrier and substrate can be difficult to control because the air has a relatively low viscosity.
U.S. Pat. No. 4,869,779 to Acheson discloses directing planarizing liquid upwardly from beneath the polishing pad against the front surface of the substrate. One drawback with this approach is that the carrier that supports the substrate relative to the polishing pad “floats” over the polishing pad. Accordingly, the apparatus cannot easily vary the downward force between the substrate and the polishing pad.
U.S. Pat. No. 4,256,535 to Banks discloses placing a drop of water between the rear surface of the substrate and the carrier. One drawback with this approach is that the water can be squeezed out from between the substrate and the carrier during planarization. Accordingly, the carrier can contact the rear surface of the substrate and abrade material from the substrate. The abraded material can then become caught between the downwardly facing surface of the substrate and the planarizing pad, potentially scratching the surface of the substrate. Another drawback is that the substrate rotates freely relative to the holder, which can reduce control over the motion of the substrate.
U.S. Pat. No. 4,373,991, also to Banks, discloses directing a continuous flow of pressurized water against the backside of the substrate. One problem with this approach is that the substrate carrier rotates relative to its support arm, increasing the mechanical complexity of the apparatus, as discussed above.